Polarization diversity system



Feb. 25, 1969 M. KATZlN POLARIZATION DIVERSITY SYSTEM Original FiledJuly 10, 1962 Sheet 1 of6 Feb. 25, 1969 M. KATZIN 3,430,156

POLARIZATION DIVERSITY SYSTEM Original Filed July 10, 1962 Sheet 2 of s1 Claim ABSTRACT OF THE DISCLOSURE A radar system in which gain andphase shift is controlled by controlling helix voltages of four TWTs,two in cascade in one channel and two in cascade in another channel,parallel with the first. In each channel one TWT is arranged to controlgain and other control phase, and control is inverse in the separatechannels. Both phase and gain, or either phase or gain alone, can becontrolled relative to one another in the channels.

This application is a division of my application, Ser. No. 208,878,filed July 10, 1962, now Pat. No. 3,234,- 547, issued Feb. 8, 1966, andentitled Polarization Diversity System.

The present application relates generally to radar systems, and moreparticularly to systems for improving the signal to noise ratio of radarsystems, and improving their capability to operate accurately in thepresence of noise, decoys, clutter and the like undesired signals,wherein discrimination against the undesired signals is provided interms of difierence of polarization of the desired and undesired signal.

Briefly describing a preferred embodiment of the invention, it isassumed that a distinguishing characteristic exists as between the noisesignal and the desired signal in addition to a difference inpolarization of wave energies representing said signals. The term noiseherein is not restricted to random noise but may be any undesired signalwhich tends to degrade a desired signal. Such difference may, forexample, be represented by a band width difference, as representing onesimple type of difference, and a time of occurrence difference, asanother, but the invention is broadly applicable regardless of thecharacter of difference so long as that difference permits a sample ofthe noise to be isolated from signal plus noise.

In accordance with the invention the polarization of the noise signal issensed, and converted to a control signal. The control signal is in turnutilized to control the polarization for which a main receiver channelprovides no noise output, but some signal output, on the assumption thata polarization difference exists between the signal and the noise. Morespecifically the noise signal, having been isolated from signal plusnoise, is divided into two polarization components. These may bedenominated V and H components, for convenience, and these designationsmay indicate that vertical and horizontal polarization components may beutilized, but without detracting from the generality of the exposition,since the specific orientations selected for polarization are arbitrary.In a broad sense then V and H represent differently directedpolarization components, which may be but need not be orthogonal. V andH components of the noise are equalized in amplitude and equalized oropposed in phase, in response to control signals derived from V and Hcomparison devices, and these same control signals are utilized tocontrol the gain and the phase shift of V and H main channels whichcarry signal plus noise such that at the output of the main channels thenoise signals will be equal in amplitude and in phase, since they wereso nited States Patent 3,430,156 Patented Feb. 25, 1969 in thesupplementary or noise per se channels, but the desired signalcomponents will in general be neither equal in amplitude nor co-phasal.Accordingly by suitably combining the outputs of the main channels, thenoise signal may be balanced and a residual desired signal remain.

It is accordingly a broad object of the present invention to provide aradar system capable of distinguishing between desired signal andundesired signal on the basis of differences in the polarizations ofwave energies representative thereof.

It is another object of the present invention to provide a radar systemfor distinguishing desired signals from noise when the noise signal isconsidered to be any degrading signal having some differencecharacteristic with respect to desired signals which can bedistinguished, in addition to a difference of polarization, such as timeof occurrence or frequency, for example.

It is still another object of the invention to provide a radar systemfor distinguishing echo from noise, wherein the term noise meansundesired signal, by so treating the noise signal alone in two channelsthat the outputs of the channels may be cancelled one with the other,and similarly treating the echo plus noise in two channels, so that theoutputs of the latter channels may provide for cancellation of the noisewithout cancellation of the desired echo signal, provided that adifference of polarization exists between the echo signal and the noise.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIGURE 1 is a block diagram of a radar system according to theinvention;

FIGURE 2 is a wave form diagram useful in explaining the operation ofthe system of FIGURE 1;

FIGURES 3 and 4 are block diagrams of modifications of the system ofFIGURE 1;

FIGURE 5 is a block diagram of the system of FIG- URE 1, employingtraveling wave tube (TWT) gain and phase control systems;

FIGURE 6 is a plot of the gain versus helix voltage of a TWT;

FIGURE 7 is a plot of the phase shift versus helix voltage of a TWT;

FIGURE 8 is a block diagram of a Doppler radar system according to theinvention; and

FIGURE 9 is a block diagram of a pulse Doppler radar system, accordingto the invention.

Referring now more particularly to FIGURE 1 of the drawings, A and Arepresent two preferably orthogonally polarized antennas, A and A Forsimplicity transmitter Tx is shown feeding only one of the antennas Athrough duplexer Dx. In fact, transmitter Tx could feed antennas A and Aor any combination of these, in any amplitude and phase relation. Thereturned echo signal is assumed jammed by noise, under which is buriedthe signal of interest, i.e., the received echo. The two orthogonalcomponents of the received noise and of the received echo signal, inantennas A and A pass through variable gain amplifiers G and G andvariable phase units p and Q52, and on to the mixers M and M After beingmixed with the output of local oscillator voltage L0 in mixers M and Mthe two signal components continue on through intermediate amplifier IFand intermediate amplifier IF and are summed vectorially in additionunit ADD. The sum signal leaves the addition unit and continues on downthrough the detector and video amplifier DV, to oscilloscope T, causinga vertical deflection. The two signal components leaving intermediateamplifiers IF and 1P also continue on to gates GT and GT These twosignal components can go through gate GT and gate GT only for a periodand at a time determined by the range unit R, which controls gategenerator GG Thus the outputs of gate GT and GT represent small samplesof the two received wave energy components. These two componentscontinue on down to amplitude detector DET and phase detector DET In theamplitude detector DET the two signal amplitudes are compared, and theirdifference is fed back to the two variable gain units G and G adjustingtheir respective gains so that the level of the two signal components isequal at addition unit ADD. In phase detector DET, the phase of the twosignals is compared. This comparison is then used to control variablephase units and such that the phase of the two signal components is 180out of phase at addition unit ADD. The information so developed 'byamplitude detector DET and phase detector DET is held, by theirrespective hold units H and H until the next repetition period of theradar system, when new information concerning signal amplitude and phaseis determined and the variable gain units G and G and the variable phaseunits 4: and are readjusted. The times of occurrence of the gatingpulses to gate GT and GT are controlled by range unit R, which may haveboth automatic (not shown) and manual features. In manual operation thedelay from the pulse from transmitter modulator MOD of the gating pulsesis adjusted manually with a knob or crank. In the automatic feature therange unit will automatically sweep the time delay of the pulses togating units GT and GT back and forth over some predetermined rangeinterval. During the period of gating pulses to gate GT and GT theoutput of addition unit ADD should approach zero. However, if at theperiod of these gating pulses there is only noise, but a short timelater the signal of interest occurs, there will be an output fromaddition unit ADD at a period after the gating pulses to GT and GT Inthis event, the signal occurring after the gating pulses enter rangeunit R and stops the automatic sweep. Thus the system will lock in rangeon the desired target. Systems of this type are well known, andaccordingly are not further described.

In operation, modulator MOD keys transmitter Tx and also sends a syncpulse to range unit R and to range display oscilloscope O, to start thesweep. In range unit R a pulse is developed which is delayed in rangewith respect to the modulator pulse. This delay may be manuallycontrolled, or it may sweep back and forth over a range interval ofinterest, and lock automatically on a desired signal. The output of therange unit R controls a gate generator GG to develop on gating signalsfor gates GT and GT The outputs of hold circuits H and H may proceed topolarity reversers or phase splitters P and P,, for amplitude and phaserespectively. These elements assure that the output of detectors DET andDET which may be positive or negative, are applied to gain controlcircuits G G in opposite senses, i.e., an increase to G is alwaysaccompanied by a decrease to G and vice versa, and similarly for and 4:The position of the period selected for the desired target is usuallyindicated on output oscilloscope O as a step, in practical systems. Inthe event that the target of interest has a range rate, standardcircuitry could also be used to provide an automatic range trackingsystem.

FIGURE 2 shows what might be expected to be seen on a normal A scopetype output from this type radar system. The transmitter pulse occurs attime t During the period t the noise jamming is apparent. Period 1corresponds to the time at which gates GT and GT were open. During thisperiod we have no noise jamming, assuming that the system is operatingto eliminate external noise, and there is only a little receiver setnoise on the oscilloscope. The situation remains the same through time tand during time t we see the signal of interest. In the period t we haveonly internal noise, under the assumption that the polarizationcharacteristics of the jamming noise have not changed in this time. Wesee the noise during time t because it is the longest time since theprevious repetition period, when the polarization receptioncharacteristics of the radar were modified so as to eliminate noise.

In FIGURE 1, all phase and amplitude adjustments have been made in theRF section of the receiver. These phase and amplitude adjustments,depending upon the overall band width required in the adjustmentsections, could be done equally as well at the intermediate frequency.Relative amplitude adjustments in the two channels could be accomplishedin any of the following ways:

(1) Adjustment of the gain of RF amplifier in either, or both, of thesignal channels.

(2) Adjustment of the gain in either, or both, of the intermediatefrequency amplifiers.

(3) Adjustment of the line loss in the RF portions by dissipative orreflective elements, or adjustment of line loss in the intermediatefrequency channels employing dissipative or reflective units.

Relative phase adjustment in the two channels could be accomplished inany of the following ways:

(1) Adjustment of the phase in the RF lines up to the mixers or in thelines between the local oscillator and the two mixers, through the useof ferrite or varactor assemblies. The varactor assembly would bepreferable in high speed applications.

(2) Through the utilization of the phase transfer characteristics of atravelling wave tube placed in either the RF lines or the lines betweenthe local oscillator and the two mixers.

FIGURE 1 illustrates a system which operates for signal selection on thebasis of time. That is to say, that a determination of the interferringsignal polarization is made at a specific time, and this information isheld or stored for a predetermined period during which the desiredsignal may be received. The noise and the desired signal have beenseparated on the basis of a difference in polarization, and upon theassumption that the polarization of the interferring noise has notchanged for a short period after the sampling time.

In FIGURE 3 is illustrated a system which operates on the basis offrequency, i.e. in FIGURE 3 one makes use of the polarizationdifferences between the noise jamming signal and the desired signal andalso makes use of the fact that the noise jamming signal covers a muchwider spectrum of frequencies than will the desired signal. The systemof FIGURE 3 operates very much like the system in FIGURE 1 throughintermediate frequency amplifiers IF and IF Up to the outputs of thosetwo IF amplifiers the overall bandwidth of the system is more than isrequired to pass the desired signal. For purposes of illustration, wemay assume that the bandwidth up to this point is twice the bandwidthrequired to pass the desired signal. The desired signal, therefore, isfound in only one-half of the available bandwidth, but wide band noisejamming signal may be expected across the full bandwidth. The outputs ofIF and 1P are applied to two sets of filter networks. Filter networks F8and filter network PS are adjusted to pass the signal frequency, whilefilter networks FN and F'N are adjusted outside of the bandwidthrequired for the desired signal. The outputs of filter networks FN andFN which are amplitude compared in detector DET and phase compared indetector phase detector DET, contain no components of desired signal.The outputs of amplitude detector DET and phase detector DET are used tocontrol variable gain units G and G and variable phase units (i and in amanner similar to that described for the system in FIGURE 1, so that theoutputs of the noise signal components at the input to filter network F8and PS are equal in amplitude and out of phase. The outputs of filternetworks PS and PS continue on through delay units D and D and are addedvectorially in addition unit ADD. The output of the addition unit ADD isdetected and amplified in detector and video amplifier DV. In additionunit ADD, since the noise components of the two channels are equal inamplitude and 180 out of phase, the noise cancels, and only the desiredsignal is left as output for presentation on output oscilloscope 0.Since the amplitude and phase corrections for the two channels cannot bemade in zero time, the output of signal of filter F8 and filter PS wouldbe contaminated to some degree by noise which got through the systembefore the phase and amplitude adjustments could be accomplished, unlessdelays were provided. The delays provided by delay units D and D arejust sufficient to minimize such noise. An alternative way of obviatingthis difiiculty is by means of the circuit shown in FIGURE 4. In thissystem variable gain unit G and variable gain unit G along with variablephase unit and variable phase unit have been inserted into the IF lines.The outputs of intermediate amplifier TF and intermediate amplifier IFsplit into two channels going through two sets of filters FS PS and FNFN as in FIGURE 3. Amplitude detector DET and phase detector DET, nowcontrol the variable gain units and variable phase units in the IF linein a manner similar to that in FIGURE 3. However, because of theinsertion of delay line D and delay line D before filter F5 and filterPS we have compensated for the delay in adjusting phase and amplitude;that is to say, the variable gain in phase units qs and 5 have beencontrolled by the respective detectors before the signal which would becontaminated with noise arrives at this portion of the system. In thisway the cancellation in the summation unit ADD will be complete and onlythe desired signal will be available for presentation on outputoscilloscope O. The variable gain and phase units have been shifted intothe intermediate frequency lines here to show that they need not beoperated in the RF portion of the system as shown in FIGURES 1 and 3. Bythe same token the features of FIGURE 4, that is, a delay to compensatethe time required to adjust phase and amplitude, can be added to FIGURE3 with the variable delay occurring in the RF before the variable gainand variable phase units.

A particularly effective device for controlling gain and phase shift isthat of using a traveling wave tube to control either or both amplitudeor phase. The traveling wave tube is a good competitor as a method forcontrolling gain and/or phase due to its extremely wide bandwidth,allowing very rapid response of the overall system. The way in which thetraveling wave tube would be used can be seen quite clearly by referenceto FIGURES 6 and 7. FIGURE 6 is a plot of gain of the tube versus helixvoltage. FIGURE 7 is a plot of relative phase shift through the tubeversus helix voltage. Gain control could be accomplished by twodifferent modes of applying operating voltages on the tubes. If we referto FIGURE 1, we see that when we need more gain from G we need less gainfrom G and vice versa. Accordingly, the quiescent or steady statecondition of G could be selected as point B on FIGURE 6, while quiescentor steady state conditions for G could be selected as point C. Then anincrease in helix voltage would increase the gain in G and decrease thegain in G Reference is made to FIGURE 5 as exemplifying such a system.The converse, of course, would hold for a decrease in helix voltage. Inthe alternative, both helices may operate quiescently at point B (or C)and a phase splitter be used to shift the operating points in responseto signal, in opposite senses, as in FIGURE 1. The traveling wave tubemight be used for phase control, as illustrated in FIGURE 7. Both phasecontrollers 5 and are quiescently biased at point A. Phase detector DET,would be so arranged as to feed back voltages of opposite senses to thehelix on each tube. Thus if we require more phase shift in the lineconnected to antenna A a less positive voltage would be fed back to thehelix of variable phase tube 4 and a more positive voltage fed back tothe helix of variable phase tube Reference is made to the June 1962issue of Microwave Journal, page 99, wherein is provided an article byDe wirs and Swarner, describing a suitable varacter phase shiftcontroller. Diode amplitude controls and ferrite phase shifters are soldcommercially by Melabs Corporation.

The system of FIGURE 8 illustrates the system of the invention asapplied to a Doppler radar system. In such a system transmission ofelectromagnetic microwaves occurs continuously at a frequency fgenerated by oscillator 30, via antenna 31. The continuous output ofoscillator 30 is converted in modulator 32, to a frequency 13-13, the ffrequency being supplied by oscillator 33.

The reference numeral denotes an antenna, preferably of the horn type,which feeds a polarizer 41 adapted to separate the signal received bythe horn antenna 40 into vertical and horizontal polarization componentson leads or channels 42 and 43, respectively. Assuming that the receivedsignal derives from a moving target the received frequency will be f +fwhere is a Doppler component. The components fq-I-f proceed to mixers35, 36, where they are mixed with frequency f h, to provide outputs onleads 42, 43, at frequency f +f f may be a relatively low frequency,comparable with a typical intermediate frequency used in asuperheterodyne radar receiver, such as 30 me. The signals on leads 42,43 are composed of vertical components V V and horizontal components Hand H respectively, where subscript 1 applies to signal and subscript 2to noise. A noise signal may be assumed due to a target which is notmoving at adequate velocity, so that the system is capable ofdiscriminating between targets on the basis of a difference in theirvelocities, and the polarizations of their returned signal. Thedifferences of their velocities provides the discriminatory basis onwhich the polarization discriminating circuits can operate. It will beunderstood that the utilization of vertical and horizontal components isa matter of choice, representing a convenient mode of describing andprocessing cross polarized signals. The vertically polarized componentof the complete received signal, v and v is applied in parallel tosignal selectors 44 and 45, of which signal selector 44 responds only toa noise component v of the total received signal. In the case of a decoytarget, having a Doppler frequency other than I the signal selector 44may be a narrow band filter or receiver, which is so tuned as to respondto received frequencies not pertaining to Doppler frequencies of thedesired target. So selector 44 may respond to frequency f and selector45 to f +f for example.

Similarly a horizontally polarized component of the entire receivedsignal, H +H applied to channel 43 is separated in selectors 46 and 47respectively, the signal selector 46 abstracting the horizontallypolarized component of the noise H i.e. signal at frequency f;, whilethe signal selector 47 passes the horizontally polarized component ofboth noise and desired signal, H and H i.e. signal at frequency f +fConsidering now the noise channels alone, the V and H components of thenoise are passed through controllable attenuators 48 and 49, the outputsof the attenuators 48 and 49 consisting of power dividers 50 and 51,

respectively. Power divider 50 supplies a portion of the signal suppliedthereto to an amplitude detector 52 while power divider 51 supplies thelike portion of the signals applied thereto by the controllableattenuator 49, to an amplitude detector 53. The detectors 52 and 53supply their outputs to a difference amplifier 54, the output of whichconsists of two D-C signals on leads and 56, re spectively, thesesignals representing the difference of the detected input to thedifference amplifier 54 taken in opposite senses. If the V and Hcomponents are equal, the detectors 52 and 53 will be supplied withsignals of equal amplitude and the output of the difference amplifier 54as seen on the leads 55 and 56 will be zero. In such case theattenuators 48 and 49 will be subjected to zero level control signals,and accordingly will introduce a normal or zero level attenuation. If onthe other hand the output of the detector 52 is the larger and theoutput of the detector 53 the smaller, indicating that the V componentis larger and the H component smaller, signal on the lead 55 will bepositively going, and on the lead 56 negatively going, with respect tothe zero level, whereby the controllable attenuators 48 and 49 will beadjusted in respect to gain in opposite sense, and the control will besuch as to reduce the output of the differential amplifiers to zero. Itwill be clear from the discussion that attenuation may be positive ornegative, with respect to reference level. In a sense, then, thedifferential amplifier 54 is the error detector of a servo system, whichtends to reduce amplitude to zero. If the H component should be greaterthan the V component, the differential output from the differentialamplifier 54 will be positive on lead 56 and negative on lead 55, sothat attenuation introduced in the channels will be again in oppositesenses such as to tend to equalize the outputs of the attenuators.

The equalized V and H signals are now applied to controlled phaseshifters 60 and 61, respectively. The outputs of the latter are appliedto a phase detector 62, which supplies to an amplifier 63 a signalrepresentative of the phase difference of the two inputs of the phasedetector 62. The amplifier 63 is arranged to provide oppositely phasedD-C control signals to the controlled phase shifters 60 and 61, vialeads 65 and 66, respectively, in opposite sense, so as to tend toequalize the phases of the output signals derivable from the controlledphase shifters 60 and 61. The phase detector 62 is then again the errordetector of a servo, wherein the character of the error is a phaseerror, and wherein the servo loop tends to reduce the error to zeroregardless of its sign.

There is now available on leads 55 and 56 two control signals, whichrepresent the change in gain which must be introduced to the V +V signalsupplied by the signal selector 45 and to the H +H signal supplied bythe signal selector 47, in order to equalize the V and H components ofthe latter signals. These control signals, available on leads 55 and 56are supplied to controllable attenuators 70 and 71, which duplicate thecontrollable attenuators 48 and 49. It then follows that at the outputsof the attenuators 70 and 71, i.e. on leads 72 and 73 respectively, aretwo signals V +V and H +H respectively, wherein the V and H componentsare equal, but wherein, in general, V and H components are not equal,and would only be equal if the polarization of the desired signal werethe same as the polarization of the noise signal.

The amplitude equalized signals available on leads 72 and 73 are appliedto controlled phase shifters 74 and 75, which duplicate, respectively,the controlled phase shifters 60 and 61, and which are in factcontrolled by the same control signals as are the latter, via leads 65and 66. Accordingly, at the output of the controlled phase shifters 74and 75, i.e. on leads 76 and 77, appear the signals V +V and H +Hrespectively, but the V and H components of these signals are equalizedin amplitude and in phase, whereas the V and H components are, ingeneral, not equalized. The signals on channels 76 and 77 are applied toa difference amplifier, 78, which serves to cancel the V and Hcomponents of the total signal applied thereto, and to combinealgebraically the V and H components, providing an output in response tothe latter on lead 79.

In order to clarify the operation of the system of FIG- URE 8, to thispoint, by way of example, it may be assumed that a noise signal isintercepted, which is plane polarized 45 from the vertical takenclockwise, whereas a desired signal simultaneously intercepted is planepolarized at 90 with respect to the noise signal, i.e. at 45 takencounterclockwise to the vertical. In this set of circumstances, the Vand H components will be equal, in

both channels, and accordingly no control signal need be supplied to thecontrollable attenuators 48 and 70, 49 and 71. Similarly, the phases ofthe noise signal components V and H may be equal, so that no phase shiftcontrol signal need be supplied. Therefore, no control is required orobtainable from the system for the specific set of circumstancesspecified, i.e. V and H equal and co-phasal. So far as the signal plusnoise channels are concerned, the V and H components pass throughwithout modification of either amplitude or phase, these signals beingco-phasal and of equal amplitude at the output terminal. The V and Hsignals are likewise of equal amplitudes, since they represent 45polarization. However, since the polarization is counterclockwisepolarization, the phase relation of the V and H components are oppositeinstead of equal. Accordingly the difference amplifier 78 provides twicethe output available on either input channels 76 or 77 alone, and amaximum output signal representative of the desired signal to theexclusion of the noise signal becomes available on output lead 79.

For any set of circumstances conceivable, i.e. whether or not one or theother polarizations is rotating, or if both are rotating, whether therotation is in the same sense or in opposite sense, an output signalappears from the difference amplifier 78 whenever the polarization ofthe two signals is not the same.

The control effects, in the several channels, involving control ofamplitude and phase of V and H components can be arranged to take placein substantially real time, implying that these occur substantiallyinstantaneously. In terms of current state of the art capabilities, thiscontrol can be effected in a few mill-microseconds. Such rapidity ofresponse enables the system to achieve and maintain differentialresponse as between signal and noise, in the case of pulse radarsystems, for example, substantially throughout each pulse.

This capability is important in maintaining the discrimination againstthe noise in spite of variations in the polarization characteristics ofsaid noise with time.

The output of differential amplifier 78 on lead 79 is applied to adetector 80, to which is also applied the output of oscillator 33, i.e.f Since the input to detector 80 is at frequency f +f the output may beat frequency 13;, on lead 81.

In order to assure that the polarization control system may have time toact, a slight delay may be introduced following filters 44, 45, in termsof delay units 82 and 83.

While the system of FIGURE 8 has been described as utilizing Dopplerfrequencies below a desired range of values to eliminate decoy targets,the desired range pertaining to desired targets, i.e. those movingwithin a desired range of velocities, the philosophy of the systemequally permits the treatment of signals deriving from targets moving atgreater than a specified speed, as noise or undesired signal.

While the various systems exemplifying the invention have been describedas radar systems, the concepts involved are applicable to ladar systems,i.e. those employing coherent light or infra-red wave energy. Sources ofsuch coherent wave energy are called lasers.

In the event a decoy target is sufficiently different in velocity thatits Doppler frequency can be removed by filtering from the Dopplerfrequency of a desired target it might appear that the present inventionis useless. However, even in this case the signal to noise ratio of thedesired signal may be improved, since there may be Doppler spectraderiving from the true and decoy targets, and these spectra may overlap.

The more useful situation occurs when a cloud of decoys is employed, allhaving similar polarizations but different velocities. In such case,some of the decoys may have the same velocity as, but differentpolarizations than, the true target, and the effect of these on thedesired signal can be eliminated.

The system of FIGURE 1 may be modified as in FIG- URE 9, to provide apulsed Doppler radar system, improved according to the principles of thepresent invention.

The pulsed Doppler radar system differs from the more conventional radarsystem in that the transmitted pulses and the local oscillator signalsare both derived from a common master oscillator, so that they arecoherent and locked.

According to the invention, a master oscillator 70 provides wave energyat frequency f to a coherent transmitter Tx which is pulsed by amodulator MOD. The frequency f is then the transmitted frequency,passing via duplexer Dx to antenna A as in FIGURE 1. The modulator MODalso provides sync pulse to the oscilloscopic indicator 0.

The output of oscillator 70, at frequency f may then be converted tolocal oscillator frequency f f by mixing with the output of oscillator75, at frequency f in a mixer 76. The output of addition unit ADD,corresponding with unit ADD of FIGURE 1, will contain a Dopplercomponent i which can be detected by mixing in detector 77 withfrequency f deriving from oscillator 75. The nominal IF frequency of thesystem is 11, since f is the carrier frequency and f f the localoscillator frequency, but since f in the return signal acquires aDoppler component 13;, the IF frequency in fact is f +f responsive to amoving target. The frequency i selected from the output of detector 77for desired target velocities, and applied to the video terminal ofoscilloscope indicator 0, enables range to be presented for any targetvelocity or velocities, but excludes indications from non-movingtargets.

A considerable number of MTI, or moving target indication, radars aredisclosed in chapter 16 of Radiation Laboratory Series 1, by Ridenour,entitled Radar System Engineering, published by McGraw-Hill. It Will beobvious that any of these systems lend themselves to modification,according to the principles of the present inven tion as exemplified inany of FIGURES 1, 3, 4, 7, 8, 9. The modification of FIGURE 1, toproduce FIGURE 9 is accordingly desired to be non-limiting, but ratherexemplary, of the invention as applied to pulsed Doppler systems.

What I claim is:

1. In a system for controlling the relative gains and phases of a commonsignal applied in two channels, a first traveling wave tube amplifierconnected in said first channel, a second traveling Wave tube amplifierconnected in cascade with said first traveling wave tube in said firstchannel amplifier, a third traveling Wave tube amplifier connected insaid second channel, a fourth traveling wave tube amplifier connected incascade with said third traveling wave tube amplifier in said secondchannel, said traveling wave tube amplifiers each having a dome-shapedgain versus helix voltage characteristic including a relatively flatpeak and oppositely sharply sloping sides descending from said peak,means for setting helix voltage operating points for said first andthird traveling wave tubes symmetrically on opposite ones of saidsloping sides, said traveling wave tubes having a linear phase shiftwith said helix voltage characteristic, and means for setting helixvoltage operating points for said second and fourth traveling wave tubesat said peak, and means for applying a common input signal to both saidchannels for transfer by said traveling Wave tube amplifiers, wherebychange in phase is unaccompanied, in said second and fourth travelingWave tube, by a change in amplitude.

References Cited UNITED STATES PATENTS 2/1960 Cutter 3l53.6 X 3/1962'Cicchetti 3l53.6 X

NATHAN KAUFMAN, Primary Examiner.

